This invention relates to mixers, and more particularly to high frequency mixers.
Electromagnetic waves in the high frequency band have a characteristic such that the physical dimensions and geometry of circuit hardware is very often as important as and sometimes more important than the electrical circuit configuration. Moreover, at such frequencies optimization of either a physical or an electrical parameter usually occurs at the expense of another parameter. Thus, a blending and combining of hardware and electrical circuit aspects of a radio frequency device becomes a challenge.
The invention provides a high frequency mixer wherein the conversion loss is significantly less than it has been in prior art devices, yet this reduction has been accomplished without adversely effecting other performance characteristics. For example, prior to the invention, a reasonably expected conversion loss for a state-of-the-art cable television (CATV) mixer was in the order of 6-10 dB. The invention has successfully reduced that loss to the 3.5-4.5 dB range, over nearly a full octave of local oscillator (LO) frequency range, for a CATV input frequency band of 50-550 MHz. The isolation between the LO input port and the RF/IF (radio frequency/intermediate frequency) port exceeded 40 dB for greater than 10 dBm LO injection. The third order intercept was better than +20 dBm.
An important consideration in achieving low conversion loss is the on/off times of the diodes and the resulting shape of the mixer switching waveform. A shorter switch-on time, with a fast and clean switching waveform, provides a lower conversion loss. However, a shortened switching time increases the impedance of the mixer and makes stray reactances more critical. In this inventive mixer, the diodes are driven so as to switch simultaneously and also to appear at the LO port in parallel rather than in series. This is accomplished by means of a cross-coupling arrangement at the LO port, which reduces the mixer LO impedance by a factor of four.
A theoretical analysis shows that with a +15 dBm injection and a diode reverse-bias of about 1.5V. per diode, and with full recovery of all undesired products, the VHF or UHF conversion loss of a mixer should be about 3 dB. The measured performance of the inventive mixer was found to be close to theoretical, i.e. in the range of 3.5 to 4.5 dB.
To assure clean, sharp short diode switching transitions, it is necessary to control the magnitude and phase of all harmonics of the LO signal, which in turn requires minimization and control of stray reactances. The invention achieves these desirable features by providing a very compact and symmetrical physical construction of all components, as well as by integrating the mixer diodes in association with shielded lines, the diodes being cross-coupled at the LO side of the mixer so as to appear in parallel to the LO source. In order to control distortion, the switching waveform should not be affected by the RF signal. Therefore an LO-to-RF signal ratio larger than 40 dB should be used. Except for a small ferrite balun between the local oscillator and the inventive mixer, there are no transformers or baluns that would contribute to conversion loss and imbalance.
The result is a new and improved high frequency mixer having an extremely low conversion loss without a countervailing reduction in other performance characteristics. The resulting mixer also combines mechanical geometry and electrical circuitry to provide a low cost, high performance, single-balanced or double-balanced device.